1. Field of the Invention
The present invention relates generally to a method of bonding substrates to a thermoplastic base. More specifically, the present invention is a method of utilizing heat and pressure to form a permanent bond between a foam or substrate, such as polyethylene, and a thermoplastic base. The foam is controllably melted while in direct contact with the base and then subsequently allowed to cool and solidify, thus forming a permanent bond.
2. Description of the Related Art
Plastic has become the material of choice for a plethora of applications due to its ability to be easily formed or molded into a desired product. It is easy to work with, reliable, cost effective, and durable. Most importantly for many medical applications, plastic is safe for most human interactions. Additionally, plastic products can be routinely manufactured in bulk, thus achieving savings which will make the final product more cost effective, and hence more marketable.
Due to the ability to make consistent, high quality products relatively inexpensively, it has become fairly common to provide the end user with a bulk supply of an item; thus allowing for a single use of a particular product. In other words, a quantity of disposable items is provided rather than a single, reusable item. Often times this is advantageous in that reusing a product, especially for medical purposes, may be unsanitary or simply impractical.
One common way of making products out of plastic is to use an injection molding process. A polymer thermoplastic is heated into a liquid state and then injected into a rigid mold. The mold forms the outer periphery of the shape of the completed product. The plastic is then allowed to harden and a completed product is achieved. This is a very effective method of mass producing plastic products and allows for an infinite number of design possibilities. One particular advantage of injection molding is the ability to make relatively small components. Another advantage is the ability to disperse the plastic into a relatively thin substrate, thus forming a sturdy, yet flexible member.
One drawback to the use of some thermoplastics is the inability to adhere other materials to the completed product. The nature of the plastic used precludes an effective use of most, if not all known adhesive products. The low surface energy of the plastic, combined with the oils present in the plastic, cause even the most aggressive adhesives to separate with a minimum force. As such, a real difficulty is presented if the attachment of another component is necessary.
Manufacturers and end users have been unable to find a solution that allows a component or substrate to be adhered to a product made of thermoplastic. One way of avoiding the problem has been to configure the thermoplastic product such that an interlock is created. For example, a molded automobile dash generally covers a layer of foam. The foam cannot easily be adhered to the dash, so the dash is shaped to partially encase the foam. While this is an adequate solution in some situations, the concept cannot be employed with most other products.
Often the product itself must be uniquely configured to carry out its particular purpose. For example, the inventor has developed an incontinence device for females. The device consists of a generally flat, teardrop shaped plastic membrane made from thermoplastic. The membrane has a flexible tubular protuberance that snaps between an open and closed position. A pullstring is attached to the flexible portion of the device. The protuberance has several small holes and acts as a simple valve. In the open position, fluid is allowed to flow through the holes. In the closed position, the flexible portion is pushed against the remainder of the protuberance, thus preventing fluid from passing through the holes.
The device is placed and retained over the meatus of the female urethra. Urine is prevented from passing through when the device is in a closed position. When desired, the pull string is pulled, causing the valve to open. The device represents a comfortable and cost effective solution to a very embarrassing problem for a large number of females. Women who suffer from incontinence often find their lives seriously disrupted. Due to both the embarrassment which would occur from a public accidental voiding, and the perceived risk of such an event, many women feel they have no choice but to forgo many or most activities in public places. The device is safe, easy to use, and very reliable. As such, it gives incontinent women sufficient confidence to resume the normal course of their lives.
The main body of this device is made using injection-molding techniques to appropriately form thermoplastics. The injection molding process provides a fully functional valve, coupled to a teardrop shaped membrane. However, there is no way to effectively attach the plastic valve to the body. Ideally, an adhesive could be used to form a tight seal with the patient. Also, the adhesive must be separable from human skin with little or no irritation as well as being able to interact with bodily fluids. As mentioned above, however, adhesives applied to the plastic will simply separate as the slightest amount of force is applied. This dictates that less aggressive adhesives must be utilized, which are even less effective in bonding to thermoplastics.
As can be imagined, unintentional separation of the product would have a disastrous effect both for the woman attempting to use the device and the company trying to market it. Obviously for a woman to use such a device and venture into public, she must have a certain degree of confidence in the product. If the adhesive separates (as it inevitably would) and the woman is embarrassed, it would be extremely difficult and most likely impossible to ever regain that woman""s trust in the product and possibly even the company making the product.
As such, there exists a need to provide a method of adhering material to a thermoplastic substrate. In particular, there exists a need of providing an adhesive layer on a thermoplastic substrate.
The present invention utilizes heat and pressure to permanently bond an intermediate substrate, such as polyethylene, directly to a thermoplastic substrate. The polyethylene substrate provides a layer to which virtually any other item, including adhesives, can be affixed with ease and reliability.
The thermoplastic product is placed on a heating fixture that heats the underside (as positioned) of the plastic. The polyethylene layer (intermediate substrate) is placed on top of the thermoplastic product, opposite the heating fixture. A press is then lowered, sandwiching the intermediate substrate and providing a predetermined amount of pressure. The heating fixture is then maintained at a temperature slightly below the melting point of the thermoplastic (preferably about 10xc2x0 F. below) and slightly above the melting point of the polyethylene.
As the pressure is increased by the press, the heating of the layers is accelerated. The intermediate substrate acts as an insulator, trapping the heat between itself and the thermoplastic product. This causes the lower portion, but only the lower portion, of the intermediate substrate to melt while the press forces this melted material against the thermoplastic. As is known, thermoplastic softens as it is heated. As such, the softened thermoplastic product interacts with the partially melted intermediate substrate. Pressure is then removed by raising the press. The product is then allowed to cool. As it cools, the melted intermediate substrate permanently bonds to the softened thermoplastic. Since only the lower portion of the intermediate substrate melted, the remainder remains perfectly intact. As such, other products can now be placed atop the intermediate substrate. This alleviates the aforementioned problems in bonding adhesives to thermoplastics by providing the intermediate substrate to which the adhesive can be applied. The bond formed between the intermediate substrate and the thermoplastic is permanent and incredibly strong. Forcibly pulling apart the layers results in a tearing of the intermediate substrate, not in a separation of the bond therebetween.
With the incontinence device, the intermediate substrate is made up of polyethylene. This layer of polyethylene acts as a padding that makes the product even more comfortable. While the thermoplastic used is relatively pliable (specifically, a styrene ethylene-butylene styrene modified block copolymer), this additional level of comfort is beneficial. As previously discussed, the adhesive applied to this product must securely bind to the skin, but upon separation it should not create undue irritation. One appropriate product is a water-based hydrogel. The final product then comprises a layer of thermoplastic bonded to a layer of polyethylene, with a layer of hydrogel applied over the polyethylene. Covering the hydrogel is a release layer, which is simply peeled away before use, exposing the tacky hydrogel.
Applying each of these layers separately to such a relatively small component would be cumbersome and costly. Therefore, large sheets of polyethylene are provided and are uniformly precoated on one surface with the hydrogel. Subsequently, a large sheet of the release layer is placed atop the hydrogel. This layered product is then die cut to match the teardrop shape of the incontinence device.
As mentioned, hydrogel is water based. Therefore if it is subjected to heat it will rapidly evaporate which eliminates its effectiveness. In the method of the present invention, the layered product is placed atop the thermoplastic valve. The unheated press is applied against the release layer, while heat is applied to the underside of the thermoplastic. In this way, the release layer and hence the hydrogel are never directly heated. The additional advantage of this process is that the heating of the hydrogel is avoided by exploiting the insulating characteristics of polyethylene. Not only does this insulative effect prevent the upper portion of the polyethylene from melting, it prevents any significant amount of heat from reaching the hydrogel.
Materials other than polyethylene can be utilized equally well so long as the materials are chosen so that their melting point is below that of the thermoplastic used, and ideally it is relatively close to that of the thermoplastic. The present application presents this method with reference to creating an incontinence device, however the method is applicable to bonding a substrate to a thermoplastic for any purpose. The amount of time and the amount of pressure applied are product dependent variables. Sufficient time must be permitted to melt a sufficient quantity of the substrate, while too much time could eliminate the insulative benefits provided by the substrate. The pressure applied serves to accelerate this process, as the correlation of heat to pressure is well understood. As such, the level of pressure applied should be selected so as to not damage or permanently deform any of the elements being joined.
It is an object of the present invention to provide a method of permanently bonding a substrate to a thermoplastic product.
It is another object of the present invention to provide a method of placing and retaining an adhesive on a thermoplastic product.
It is yet still another object of the present invention to provide a method of utilizing heat to bond a substrate to a thermoplastic product.
It is still another object of the present invention to provide a method of utilizing heat and pressure to bond a substrate to a thermoplastic product.
It is still a further object of the present invention to provide a method for utilizing heat to bond a substrate to a thermoplastic product wherein the substrate includes a sensitive material that is shielded from the heat.
It is still yet a further object of the present invention to provide a method of bonding a polyethylene layer bearing a hydrogel adhesive onto a thermoplastic incontinence device.